16f88 - timing-critical function for serial comm

[rdxbam]

I read on this lise that PICs have a "crappy" internal oscillator in regards to doing timing-critical functions such as serial communications.

so what is a good replacement part for the internal oscillator on a 16f88?

[mackrackit]

A three pin resonator works very well for external OSC.
But..
Microchip is claiming the internal on the 16F88 IS good enough for serial. So far I have to agree with them on this part. Other parts are not very good though...

[tenaja]

rdxbam, you have heard correctly. "Some" of the time, the intosc will work (which is fine for hobbyists), but you cannot count on it--especially at higher speeds. M'chips spec is that they calibrate it within 1%, but my experience is to the contrary--I use enough of them to know they are regularly 3% or more out of spec, and I have seen them as bad as 6% off.

[mackrackit]

rdxbam, you have heard correctly. "Some" of the time, the intosc will work (which is fine for hobbyists), but you cannot count on it--especially at higher speeds. M'chips spec is that they calibrate it within 1%, but my experience is to the contrary--I use enough of them to know they are regularly 3% or more out of spec, and I have seen them as bad as 6% off.
Normally I would agree, but MicroChip seems to have done something different with this chip.
From the F88 data sheet.


Addressable Universal Synchronous
Asynchronous Receiver Transmitter
(AUSART/SCI) with 9-bit address detection:
- RS-232 operation using internal oscillator
(no external crystal required)
Like I said before, seems like they are correct. No mention of 1% +-.

Oh, and why do you think the hobby project does not have to work all of the time?

[tenaja]

For this particular part, DS section on Electrical Characteristics 18.3 says +/- 1% typical, but as bad as +/-2 Max. I believe some of the DS's say 1% in the text of the Internal Oscillator Block section (4.5 for this one), but this particular one does not.

Hobbyists can deal with manually tuning OSCTUNE, since they are dealing with a few parts here and there, and most of the time it doesn't matter. That's not quite the same as a production environment, when all of them have to work, no matter what, and there are hundreds every day. As a hobbyist, I can throw one out (or put it in a different project) or manually tune it myself. As a manufacturer, I want less than 1% scrap, without manually tuning them. Unfortunately, M'chip does not consistently meet their spec in this regard.

[Archangel]

I would add: you are not likely to sue yourself if failure occurs. The public ? Oh yea!

[rdxbam]

Thanks all...and a 20MHz 3-pin resonator it will be.

Thanks again for the education!